Power calculation: This calculation is based on the end point "attachment level (AL)", either it is measured on roentgenograms (RAL) or clinically (CAL) which again is the measurable basis for "recurrence of periodontal disease" = further loss of attachment after treatment. Based on results from similar published studies we assume that the standard deviation of AL in each treatment group will be approximately 1.5 mm. A two-sample t-test will be applied when comparing average AL in two treatment groups at a given time point, with a 5% significance level. Further, we consider differences in average AL between two treatment groups to be clinically significant if it is at least 1.0 mm.